Influenza type B viruses have been isolated during periods of widespread influenza activity during five of the last ten influenza seasons in the United States. During this time, each major peak of influenza B activity has been associated with the emergence of a new antigenic variant of the virus. As with influenza type A viruses, antigenic drift in influenza type B virus occurs through the accumulation of amino acid changes in the HA1 subunit of the major vital structural glycoprotein, the hemagglutinin (HA).
Analysis of antigenic drift variants of influenza A using molecular modeling techniques has shown that, although the major antigenic domains are clearly identified, nearly the entire outer surface of the globular head region of the HA1 subunit has changed in 10 years. While the structure of the HA of influenza type B viruses appears to be analogous to that of the type A virus HA, the spatial arrangement of the antibody combining sites on the HA of influenza B virus are undetermined since the three-dimensional structure of this protein is uncertain.
Antigenic sites on the HA of influenza type B have been delineated by sequence analysis of both circulating viruses and laboratory derived variants and may not be identical to those of influenza type A viruses. Data from sequencing studies, in conjunction with routine antigenic analysis, helped determine the evolutionary relationships between currently circulating strains of influenza virus. For recent influenza B viruses, at least two main lineages have co-existed since at least 1983. Viruses from each of these two lineages, B/Yamagata/16/88 (B/YM/88) and B/Victoria/2/87 (B/VI/87), had as many as 27 amino acid differences between their HA1 proteins by 1988 and were distinct antigenically. The B/YM/88-like viruses were found to be descendants of viruses similar to an earlier reference strain, B/USSR/100/83, while the B/VI/87-like viruses were related to the more recent epidemic variant, B/Ann Arbor/1/86.
During 1989-1990, influenza B viruses that were antigenically related to both B/VI/87 and B/YM/88 were isolated sporadically throughout the world and comprised less than 1% of the isolates reported in the United States. In spite of this limited circulation, antigenic drift variants were identified from each lineage. During 1990-1991, the majority of influenza isolates in many countries were type B. In the United States, approximately 90% of the reported influenza isolates were type B. Though viruses closely related to both B/VI/87 and B/YM/88 were identified, the majority of these influenza B isolates were antigenically related to a drift variant of B/YM/88, B/Hong Kong/22/89.
Current influenza vaccines typically comprise inactivated whole virus or "split" virus. Split virus is a membrane extract of a whole virus preparation and is usually used in children. The vaccine is produced by growing the virus in eggs, concentrating and purifying the virus, followed by inactivation with formalin. Presently, the vaccine is trivalent and comprises two type A viruses (H3N2 and H1N1 subtypes) and a type B component. B/Hong Kong/22/89, a virus whose sequence is described in this invention, is currently the type B component. The current dose is 45 micrograms (.mu.g) or 15 .mu.g of each component and the vaccine is administered intramuscularly. Influenza vaccine is typically given to high risk individuals every year in the late fall. High risk individuals include the elderly and those with chronic upper respiratory tract illness.
Because of antigenic drift of the virus, the viruses used as vaccine components are updated frequently, typically every 2 to 3 years, based on epidemiological data. The invention includes sequence data and antigenic data that can be used to describe a series of field isolates of influenza B. The sequence data would allow, for example, verification that the virus in the vaccine is in fact, B/Hong Kong/22/89. The vaccine manufacturers do not usually sequence their vaccine seed stocks.
The invention characterizes, by molecular and serologic methods, influenza B viruses representative of viruses isolated during the 1989 to 1990 and 1990 to 1991 influenza seasons. Amino acid changes potentially involved in altered antigenic reactivity are identified for both the B/VI/87-like and B/YM/88-like viruses and the genetic relationships between the HA of these drift variants are analyzed. Nucleic acid sequences and amino acid sequences for the viral strains are provided. Methods of vaccination and diagnosis are also included.